Gunmen in the north of Mexico today shot dead three family members of the police officer being held by authorities over an arson attack on a Monterrey casino in August.

The officer’s father, his stepmother, and his stepbrother were all killed in Monterrey. The authorities believe that this was an act of revenge from the influential Zetas drugs cartel - a response to the officer naming gang members that were allegedly involved in last month’s casino attack.

The blaze saw 52 people slain and caused widespread revulsion across Mexico. Thus far, Miguel Angel Barraza, the police officer in question, is one of six suspects that are detained in connection with the arson attack on Monterrey’s Casino Royale.

The other five are allegedly members of Zetas, which is believed to have torched the casino’s premises after its owners refused to hand them protection money.

The murder of the relatives came on what was a day of violence across Monterrey and at least 15 people, in total, were murdered.

Monterrey and its state of Nuevo Leon have witnessed rising bloodshed as Zetas and other Gulf cartels battle one another for control of key trafficking routes to America. The casino attack is viewed as the deadliest episode of violence ever since Mexico’s president Felipe Calderon launched his war on drugs gangs almost five years ago.